The present invention relates to a torque detecting apparatus, and in particular to a noncontact torque detecting apparatus for detecting a torque applied to a connecting shaft by detecting a torque inputted into an input shaft as a change in the inductance of a coil due to torsion which arises at the connecting shaft.
A torque detecting apparatus comprises: an input shaft to which a torque is inputted; an output shaft for outputting the inputted torque; a connecting shaft of a torsion bar for connecting the input shaft and the output shaft; and a coil, which surrounds the connecting shaft, the input shaft and the output shaft, for detecting a torque applied from the input shaft to the connecting shaft based on a torsional angle which arises at the connecting shaft. The input shaft and the output shaft respectively comprise a detection ring made of magnetic material on the outer circumference thereof, wherein when a torque is inputted into the input shaft, the connecting shaft is tortured, so that the relative positions of two detection rings in the circumferential direction are changed and the inductance of the coil surrounding the connecting shaft, the input shaft and the output shaft is changed. By detecting this change in the inductance as a change in the impedance of a coil connected to an AC circuit, a torque which is inputted into or outputted from the input shaft and the output shaft and applied to the connecting shaft is detected. The coil is held coaxially in a holding cylinder formed of green compact and engaged in a recess extending over the entire circumference of the inner surface of the holding cylinder.
By the way, the holding cylinder comprises on the outer surface thereof a projection for locating the holding cylinder at a predetermined position, the projection being formed over an area between the annular faces of the holding cylinder in a conventional torque detecting apparatus. The projection is a strip formed integrally with coating member made of synthetic resin for coating the holding cylinder (Japanese Patent Application Laid-Open No. 2002-13990).
A torque detecting apparatus constructed in such a manner is used in a vehicle electric power steering apparatus, for example. That is, the apparatus is used for detecting a torque which is inputted into the input shaft connected to a steering wheel for operating the vehicle by rotation of the steering wheel. Attachment of the torque detecting apparatus is effected by bringing the projection of the torque detecting apparatus into contact with the inner circumferential surface of a hollow housing constituting the electric power steering apparatus and fitting the torque detecting apparatus fixedly.
It should be noted that green compact has an electric resistance higher than that of iron, stainless steel or the like. Accordingly, by forming the holding cylinder with green compact, eddy current to be caused in the holding cylinder by current passing through the coil can be decreased and the detection accuracy of torque can be enhanced in comparison with a case where the holding cylinder is formed of iron, stainless steel or the like.
However, when an electric power steering apparatus is used, the temperature of the torque detecting apparatus and the housing is changed in a large way from ordinary temperature. When the temperature is changed, the housing and the torque detecting apparatus expand or contract in different ratios due to a difference in the coefficients of linear expansion of members constituting the housing and the torque detecting apparatus, so that a large pressure, i.e. a pressure larger than a pressure which is to be applied to the projection at ordinary temperature, is applied to the projection provided at the holding cylinder. In particular, when the temperature is low, a housing made of aluminum contracts more than the torque detecting apparatus and a large pressure is applied to the projection. When pressure in the radial direction of the holding cylinder is applied to the projection, the holding cylinder is deformed and the inductance of the coil is changed, so that the detected value of torque is changed. Thus, a conventional torque detecting apparatus has a problem that the torque detection becomes inaccurate when pressure applied to the projection for supporting the torque detecting apparatus at a predetermined position is changed.
When a torque detecting apparatus is used in an electric power steering apparatus, if a change in the temperature of the vehicle makes the torque detection inaccurate, the operations of the electric power steering apparatus for assisting steering based on the detected value of torque also becomes inaccurate, giving rise to deterioration of feeling in steering the vehicle.
There is another problem that the green compact forming the holding cylinder has a low strength and, in particular, a portion of the holding cylinder having the recess has a small thickness, so that this portion of the holding cylinder might be damaged while pressing the torque detecting apparatus into the housing and the torque detecting apparatus cannot be pressed into the housing with a large force.
When the torque detecting apparatus cannot be pressed into the housing with a large force, it is impossible to bring the projection into contact with the inner circumferential surface of the housing with a large force, so that the torque detecting apparatus cannot be attached in a stable manner.